The present invention relates to an optical disc unit and more particularly to an optical system wherein light beams from two light sources are combined and applied to an information medium, and the light beams reflect therefrom are drawn out in such a way that they are separated from each other.
Recently, an optical disc unit has been proposed in which a semiconductor laser is employed as a light source, and the laser beam therefrom is focussed into a microbeam of light not larger than 1 .mu.m and applied to a disc-shaped information medium having a photosensitive material deposited thereon by evaporation thereby to record and reproduce video signals or digital signals in the form of concentric or spiral shapes.
An example of a unit of this kind may be such that two laser beam sources are provided, and both laser beams therefrom are simultaneously focussed and applied onto the disc close to each other.
Examples of such a unit include the following: One example is such that one of the laser beams is focussed into a microspot of light of oval shape which is long in the direction of the track and is employed to erase the record in the track, while the other beam is focussed into a substantially circular microspot of light on the disc and is employed to effect recording and reproduction. Another example is such that one of the laser beams is employed to apply a thermal bias, while the other laser beam is employed to effect recording. Still a further example is such that both the laser beams are focussed into substantially circular microspots of light on the disc, and one of the microspots of light is employed to record and at the same time, the other microspot of light is employed to read out the recorded signal and reproduce the same instantaneously, thereby making it possible to check for whether a correct recording has been effected or not.
FIG. 1 shows an example of a typical conventional optical recording and reproducing unit having two light sources. More specifically, a laser beam 1a from a first light source 1 is passed through an afocal lens system 2, bent by a mirror 3 and focussed on a disc 5 by a focussing lens 4. On the other hand, a laser beam 6a emitted from a second light source 6 is passed through a lens 7 as well as a half-mirror 8, bent by the mirror 3 and focussed on the disc 5 by the focussing lens 4. Reflected light 6b from the disc 5 emanates from the half-mirror 8 and is led into a photodetector 9 for obtaining known focus and tracking control signals and a reproducing signal for reading out information recorded on the disc 5.
In such a method for composing both laser beams together while spatially keeping the beams separate from each other so that their respective optical paths do not interfere with each other, one of the laser beams (e.g., 6a) must enter at a considerably oblique angle with respect to the optical axis of the focussing lens 4, so that it is possible to effectively utilize the opening of the focussing lens 4. As a result, much light may be rejected, or an aberration may be generated resulting in deterioration of the focussing performance of the focussing lens 4.
Moreover, the semiconductor laser generally has an anisotropic divergent angle, which is different for directions perpendicular and parallel to the composition surface thereof, so that the optical system for focussing a circular microspot of light on the disc becomes undesirably complicated Accordingly, when semiconductor lasers are employed for both light sources, the optical system inevitably becomes large and complicated. In addition, since the laser beams have large diameters, it is difficult to combine both beams while keeping their optical paths spatially separate from each other.
In the arrangement shown in FIG. 1, it is also difficult to separate the reflected light of both laser beams from the disc, so that the reflected light which is emitted from the other light source (the first light source 1) may undesirably be returned and led to the photodetector 9 to exert adverse effects on the controlling performance and reproducing signals.